Ribbon crystal having reduced wetting string

ABSTRACT

A string for use in a string ribbon crystal has a base portion with a refractory material, and an externally exposed layer radially outward of the refractory material. The base portion has a coefficient of thermal expansion that is generally matched with the coefficient of thermal expansion for silicon. The externally exposed layer has a contact angle with silicon of between about 15 and 120 degrees.

PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional patent application of co-pendingU.S. patent application Ser. No. 12/200,996, filed Aug. 29, 2008, andentitled, “REDUCED WETTING STRING FOR RIBBON CRYSTAL,” and namingChristine Richardson, Daniel Doble, Richard Wallace, Weidong Huang, andScott Reitsma as inventors, the disclosure of which is incorporatedherein, in its entirety, by reference, which claims priority fromprovisional U.S. patent application No. 60/969,263, filed Aug. 31, 2007,entitled, “STRING RIBBON CRYSTAL AND STRING WITH IMPROVED EFFICIENCY,”and naming Christine Richardson, Lawrence Felton, Richard Wallace, andScott Reitsma as inventors, the disclosure of which is incorporatedherein, in its entirety, by reference.

This patent application also is related to the following copending,co-owned patent applications, incorporated herein, in their entireties,by reference: “RIBBON CRYSTAL STRING FOR INCREASING WAFER YIELD,” and“RIBBON CRYSTAL STRING WITH EXTRUDED REFRACTORY MATERIAL.”

FIELD OF THE INVENTION

The invention generally relates to string ribbon crystals and, moreparticularly, the invention also relates to string used to form stringribbon crystals.

BACKGROUND OF THE INVENTION

String ribbon crystals, such as those discussed in U.S. Pat. No.4,689,109 (issued in 1987 and naming Emanuel M. Sachs as the soleinventor), can form the basis of a variety of electronic devices. Forexample, Evergreen Solar, Inc. of Marlborough, Mass. forms solar cellsfrom conventional string ribbon crystals.

As discussed in greater detail in the noted patent, conventionalprocesses form string ribbon crystals by passing two or more stringsthrough molten silicon. The composition and nature of the string canhave a significant impact on the efficiency and, in some instances, thecost of the ultimately formed string ribbon crystal.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a string for use ina string ribbon crystal formed from a specific crystal material, such asone of silicon, silicon-germanium, gallium arsenide and indiumphosphide, has a substrate and a refractory layer supported on thesubstrate. The string also has an externally exposed layer having acontact angle with the crystal material of between about 15 and 120degrees. The externally exposed layer is radially outward of therefractory layer.

The string also may have a handling layer radially outward of therefractory layer. The handling layer applies a generally radially inwardforce to the refractory layer. The handling layer may include theexternally exposed layer. Alternatively, the externally exposed layermay be radially outward of the handling layer.

The externally exposed layer may be formed from any of a number ofmaterials that reduces wetting, such as pyrolytic carbon, oxide, andnitride. For example, the externally exposed layer may have a contactangle with the crystal material of greater than about 25 degrees.Moreover, the substrate may be formed from carbon, while the refractorylayer may be formed from silicon carbide.

Various embodiments generally match the coefficient of thermalexpansion. For example, the substrate, refractory layer, and exposedlayer have a combined coefficient of thermal expansion that issubstantially matched to the coefficient of thermal expansion of thecrystal material. To further thermal matching, the exposed layer isthinner than the refractory layer. In a more specific embodiment, thestring may have a coefficient of thermal expansion that is generallymatched to the coefficient of thermal expansion of polysilicon.

In accordance with another embodiment, a string for use in a stringribbon crystal has a base portion with a refractory material, and anexternally exposed layer radially outward of the refractory material.The base portion has a coefficient of thermal expansion that isgenerally matched with the coefficient of thermal expansion for silicon.The externally exposed layer has a contact angle with silicon of betweenabout 15 and 120 degrees.

In accordance with other embodiments, a ribbon crystal has 1) a stringwith an outer surface, and 2) a body with a body material having a bodycoefficient of thermal expansion. The body coefficient of thermalexpansion is generally matched to the coefficient of thermal expansionof the string. The string outer surface (i.e., the circumferential outersurface) also is partially exposed.

In accordance with yet other embodiments of the invention, a method offorming a string for use with a ribbon crystal forms a refractory layeron a substrate, and applies a reduced wetting material radially outwardof the refractory layer. The reduced wetting material has a contactangle with silicon of between about 15 and 120 degrees.

In accordance with still other embodiments of the invention, a method offorming a ribbon crystal provides 1) molten material having a materialcoefficient of thermal expansion and 2) a string having an outer surfacewith a contact angle with the molten material of between about 15 and120 degrees. The string also has a string coefficient of thermalexpansion that is substantially matched to the material coefficient ofthermal expansion. To form a sheet, the method passes the string throughmolten material.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages ofvarious embodiments of the invention from the following “Description ofIllustrative Embodiments,” discussed with reference to the drawingssummarized immediately below.

FIG. 1 schematically shows a string ribbon crystal that may be formedfrom strings configured in accordance with illustrative embodiments ofthe invention.

FIG. 2 schematically shows an illustrative furnace used to form stringribbon crystals.

FIG. 3A schematically shows a string formed in accordance withillustrative embodiments of the invention.

FIG. 3B schematically shows a cross-sectional view of the string of FIG.3A along line B-B in accordance with one embodiment of the invention.

FIG. 3C schematically shows a cross-sectional view of the string of

FIG. 3A along line B-B in accordance with another embodiment of theinvention.

FIG. 4A schematically shows a cross-sectional view of a ribbon crystalusing a prior art string.

FIG. 4B schematically shows a cross-sectional view of a ribbon crystalusing a string configured in accordance with illustrative embodiments ofthe invention.

FIG. 5 shows an illustrative process of forming a string ribbon crystalusing strings configured in accordance with illustrative embodiments ofthe invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a string has a reduced wetting, outerexposed layer to increase grain size near the edge of a ribbon crystal.To that end, the string may have a contact angle of between about 15 and120 degrees with the ribbon crystal material, such as single crystal ormulti-crystal silicon. To improve ribbon robustness, the coefficient ofthermal expansion of the string generally matches that of the materialforming the ribbon crystal (e.g., silicon). Details of variousembodiments are discussed below.

FIG. 1 schematically shows a string ribbon crystal 10 configured inaccordance illustrative embodiments of the invention. In a mannersimilar to other ribbon crystals, this ribbon crystal 10 has a generallyrectangular shape and a relatively large surface area on its front andback faces. For example, the ribbon crystal 10 may have a width of about3 inches, and a length of about 6 inches. As known by those skilled inthe art, the length can vary significantly. For example, in some knownprocesses, the length depends upon a furnace operator's discretion as towhere to cut the ribbon crystal 10 as it grows. In addition, the widthcan vary depending upon the separation of its two strings 12 (see FIG.2) forming the ribbon crystal width boundaries. Accordingly, discussionof specific lengths and widths are illustrative and not intended tolimit various embodiments the invention.

The thickness of the ribbon crystal 10 may vary and be very smallrelative to its length and width dimensions. For example, the stringribbon crystal 10 may have a thickness ranging from about 60 microns toabout 320 microns across its width. Despite this range, the stringribbon crystal 10 may be considered to have an average thickness acrossits length and/or width.

The ribbon crystal 10 may be formed from any of a wide variety ofmaterials (often referred to generally as “ribbon material” or “crystalmaterial”), depending upon the application. For example, when grown fora photovoltaic application, the ribbon crystal 10 may be formed from asingle element, such as silicon, or a compound, such as a silicon-basedmaterial (e.g., silicon germanium). Other illustrative ribbon materialsmay include gallium arsenide, or indium phosphide. The ribbon materialmay be any of a variety of crystal types, such as multi-crystalline,single crystalline, polycrystalline, microcrystalline orsemi-crystalline.

As known by those skilled in the art, the ribbon crystal 10 is formedfrom a pair of strings 12 generally encapsulated by the ribbon material(e.g., polysilicon). Although it is surrounded by the ribbon material(in the prior art), the string 12 and ribbon material outwardly of thestring 12 generally form the edge of the ribbon crystal 10. Forsimplicity, the ribbon crystal 10 is discussed as being formed frompolysilicon. It nevertheless should be reiterated that this discussionof polysilicon is not intended to limit all embodiments.

Illustrative embodiments grow the ribbon crystal 10 in a ribbon crystalgrowth furnace 14, such as that shown in FIG. 2. More specifically, FIG.2 schematically shows a silicon ribbon crystal growth furnace 14 thatmay be used to form the string ribbon crystal 10 in accordance withillustrative embodiments of the invention. The furnace 14 has, amongother things, a housing 16 forming a sealed interior that issubstantially free of oxygen (to prevent combustion). Instead of oxygen,the interior has some concentration of another gas, such as argon, or acombination of gasses. The housing interior also contains, among otherthings, a crucible 18 and other components for substantiallysimultaneously growing four silicon ribbon crystals 10. A feed inlet 20in the housing 16 provides a means for directing silicon feedstock tothe interior crucible 18, while an optional window 22 permits inspectionof the interior components.

As shown, the crucible 18, which is supported on an interior platformwithin the housing 16, has a substantially flat top surface. Thisembodiment of the crucible 18 has an elongated shape with a region forgrowing silicon ribbon crystals 10 in a side-by-side arrangement alongits length. In illustrative embodiments, the crucible 18 is formed fromgraphite and resistively heated to a temperature capable of maintainingsilicon above its melting point. To improve results, the crucible 18 hasa length that is much greater than its width. For example, the length ofthe crucible 18 may be three or more times greater than its s width. Ofcourse, in some embodiments, the crucible 18 is not elongated in thismanner. For example, the crucible 18 may have a somewhat square shape,or a nonrectangular shape.

As shown in FIG. 2 and discussed in greater detail below, the furnace 14has a plurality of holes 24 (shown in phantom) for receiving string 12.Specifically, the furnace 14 of FIG. 2 has eight string holes 24 forreceiving four pairs of strings 12. Each pair of strings 12 passesthrough molten silicon in the crucible 18 to form a ribbon crystal 10.

The string 12 must have a wetting angle with silicon that is low enoughto cause the molten ribbon material to adhere to its outer surface.Accordingly, some prior art ribbon crystals used in commercial solarpanels, for example, use string having a wetting angle with silicon thatis very small, such as on the order of about 11 degrees or less. Whilesufficient for many applications, those in the art noticed long ago thatsuch string creates many small grains of ribbon material in the finalproduct. Undesirably, these small grains reduce the electricalefficiency of the solar cell the crystal 10 ultimately forms.

Others have tried and failed to solve this long felt need in the art.For example, an article by Ciszek et al., entitled, “Filament MaterialsFor Edge-Supported Pulling of Silicon Sheet Crystals,” published by theSolar Energy Research Institute, Golden Colo., dated 1982 and 1983 andsubmitted with this patent application, addresses the phenomenon. Inparticular, this article discusses high contact angles of the stringwith the ribbon material (fifteen degrees or higher), but acknowledgesyield problems with materials having such high contact angles. Morespecifically, the article notes breakage problems caused by coefficientof thermal expansion mismatches between the string and ribbon material.The article does not recognize any solution to this problem.

Rather than solve the problem, the Ciszek et al. article attempts tomanage it. In particular, the article teaches use of a high contactangle (with silicon) string formed from a material having a coefficientof thermal expansion that is not matched to the ribbon material (e.g.,quartz used with silicon ribbon material). They reason that asignificant coefficient of thermal expansion mismatch will cause thestring to break off, which it suggests is a good result. Such asolution, however, is very undesirable for many applications.

Accordingly, years after the Ciszek research, the inventors discoveredhow to obtain the benefits of a string with both a matched coefficientof thermal expansion and high contact angle with the ribbon material. Tothat end, generally speaking of various embodiments, the inventorssolved of this long felt need by pursuing a solution that Ciszek teachesaway from; namely, engineering string to have both a matched coefficientof thermal expansion and high contact angle. To that end, the inventorsapplied an exterior layer of material with a favorable contact angleonto a base string portion 26 having a matched coefficient of thermalexpansion.

More specifically, FIG. 3A schematically shows a string 12 that may beformed in accordance with illustrative embodiments of the invention.FIG. 3B schematically shows a cross-sectional view of the string 12 ofFIG. 3A along cross-line B-B in accordance with one embodiment, whileFIG. 3C schematically shows a cross-sectional view of the string 12 inaccordance with another embodiment. As shown, the string 12 in bothembodiments has a base string portion 26 formed of a central core 28 anda substantially concentric, refractory material layer 30 (also referredto herein as a “refractory layer 30”).

In some embodiments, the central core 28 is a conductive carbon formedfrom conventional extrusion processes. In other embodiments, the centralcore 28 is formed from a plurality of small conductive fibers (e.g.,carbon fibers) that are wound together to form a tow. Moreover, therefractory material layer 30 may be formed from any conventionalrefractive material suitable for a given application. For example, therefractory material layer 30 may be formed from silicon carbide if usedto form a photovoltaic cell from a silicon.

Illustrative embodiments of the string 12 also have an exposed,nonwetting layer 32 that, when used with string 12 having a circular orsimilarly symmetric cross-sectional shape, also is generally concentricwith the core 28. Among other things, this nonwetting layer 32 may be acarbon, pyrolytic carbon/carbide (e.g., graphite), an oxide, or anitride. More specifically, appropriate materials may include aluminumoxide, or silica. It is preferable that the materials selected to formthis nonwetting layer 32 have no more than a negligible contaminatingimpact on the molten silicon within the crucible 18.

The inventors discovered that a very thin nonwetting layer 32 shouldminimize its impact on the coefficient of thermal expansion of the basestring portion 26. Moreover, the nonwetting layer 32 should be thickenough to provide a robust outer surface that can withstand the demandsof the ribbon crystal fabrication process. For example, a string 12having a total cross-sectional dimension of about 140 microns can have anonwetting layer 32 with a thickness of about one micron.

Although not explicitly discussed above, the string 12 also may have ahandling layer 34 radially outward of the refractory material layer 30to maintain the integrity of the base string portion 26. To that end,the handling layer 34 is formed to provide a small compressive stress tothe base string portion 26, thus improving robustness to the overallstring 12. Accordingly, if the base string portion 26 develops a crack,the compressive stress of the handling layer 34 should reduce thelikelihood that the string 12 will break. Among other things, thehandling layer 34 may be a thin layer of carbon (e.g., one or twomicrons thick for strings having generally known sizes).

The nonwetting layer 32 may be integrated directly into the handlinglayer 34, as shown in FIG. 3B. Alternative embodiments, however, mayform the nonwetting layer 32 radially outward of the handling layer 34,as shown in FIG. 3C. Yet other embodiments may form the other layersbetween the nonwetting layer 32 and the base string portion 26, or omitthe handling layer 34.

FIGS. 3B and 3C show string with generally circular cross-sectionalshapes. Various embodiments of the string 12, however, may havecross-sectional shapes that are not generally circular. For example, thestring 12 may have a concave cross-sectional shape (e.g., a cross or “c”shape), an elongated cross-sectional shape (e.g., an ellipse orrectangle), or other regular or irregular cross-sectional shape. Asdiscussed in greater detail in co-pending patent applications(identified and incorporated above), these embodiments may improve therobustness of the resulting ribbon crystals 10.

It should be noted that use of the term “nonwetting” layer is somewhatof a misnomer because if it truly was nonwetting, then the molten ribbonmaterial would not adhere to it. The nonwetting layer 32 thus can bereferred to as a reduced wetting layer, and alternatively may bereferred to in that manner below.

FIGS. 4A and 4B graphically show a primary difference between prior artstring and string 12 configured in accordance with illustrativeembodiments of the invention. Specifically, FIG. 4A schematically showsa cross-sectional view of a portion of a prior art ribbon crystal 10Pwith a prior art string 12P. This prior art ribbon crystal 10P has athin neck portion 36 between the string 12P and a wider portion 38. Asshown, the ribbon material contacts and appears to substantially coverthe entire outer surface of the string 12P. Undesirably, thissignificant surface contact produces many nucleation sites that,consequently, form a relatively high volume of small grains.

FIG. 4B schematically shows a new string 12 within a ribbon crystal 10.In a manner unlike the ribbon crystal 10P of FIG. 4A, the ribbonmaterial of this figure does not contact the entire outer surface of thestring 12. Instead, the ribbon material contacts only a portion of theouter surface, thus exposing a significant remaining portion.Accordingly, this embodiment presents fewer nucleation sites, thusfavorably reducing the number of small grains near the string 12. Inother words, this embodiment promotes larger grains near the strings 12.As a result, the efficiency of the string ribbon crystal 10 should beimproved when compared to string ribbon crystals using strings 12without a nonwetting layer 32.

Moreover, as noted above, the string 12 preferably has a diameter thatis greater than the diameters of commonly used prior art strings. Forexample, larger strings 12 may have diameters ranging from about 0.75 to2.0 times the average thickness of its corresponding string ribboncrystal 10. This larger diameter should effectively increase thethickness of the string ribbon crystal 10 in the region near the string12. Consequently, the string ribbon crystal 10 should be less prone tobreaking than some prior art designs using strings with smallerdiameters. As an example, a single crystal 10 may have a thickness thatvaries between about 140 microns and 250 microns, and the string 12 mayhave a thickness that is between 0.75 and 2.0 times such thickness.

FIG. 5 shows an illustrative process of forming a string ribbon crystal10 with strings 12 configured in accordance with illustrativeembodiments of the invention. The process begins at step 500 by formingthe core/substrate 28. As noted above, the core 28 can be formed fromcarbon by conventional spinning processes. In other embodiments,however, the core 28 may be a wire, filament, or plurality of smallconductive fibers wound together as a tow. For example, post-fabricationprocesses of a tow could form a monofilament through a known fabricationprocess, such as oxidation, carbonization, or infiltration.

After forming the core 28, the process forms a first coating/layer,which acts as the above noted refractory material layer 30 (step 502).Among other things, the first coating 30 may include silicon carbide,tungsten, or a combination of silicon carbide and tungsten. This firstlayer may be formed in a number of conventional ways, or by means of anextrusion process, a pulltrusion process, or both spinning of arefractory material with a polymer component, which subsequently isbaked off. Among other things, processes may use at least one componentof carbon, silicon, silicon carbide, silicon nitride, aluminum, mullite,silicon dioxide, BN particles, or fibers mixed with a polymer binder,coupled with extrusion/pulltrusion. This also may involve bicomponentextrusion of a core 28 with at least one silicon carbide, carbon,silicon, and a sheath with a least one of oxide, mullite, carbon, and/orsilicon carbide. Accordingly, the core 28 effectively acts as asubstrate for supporting the refractory material layer 30.

This step thus forms the base string portion 26. It should be reiteratedthat the base string portion 26 may be formed from any of a wide varietyof materials, such as a graphite fiber or tow, a refractory material,such as tungsten or silicon carbide, or a combination thereof. In fact,some embodiments may form the base string portion 26 without a core 28.

At this point in the process, the base string portion 26 has a combinedcoefficient of thermal expansion that should generally match thecoefficient of thermal expansion of the ribbon material. Specifically,the thermal expansion characteristics of the string 12 should besufficiently well matched to the ribbon material so that excessivestress does not develop at the interface. Stress is considered excessiveif the string 12 exhibits a tendency to separate from the ribbon duringreasonable subsequent ribbon crystal handling and processing steps, orif the string 12 exhibits a tendency to curl outwardly or inwardly fromthe ribbon crystal edge.

The process then continues to step 504, which forms the exposednonwetting/reduced layer 32 on the base string portion 26. As discussedabove, this layer could also serve as the handling layer 34 andpreferably is very thin so that it has a negligible impact on theoverall string coefficient of thermal expansion. For example, thereduced wetting layer 32 should be much thinner than that of therefractory material layer 30.

As also discussed above, the contact angle with the ribbon material ofthe exterior surface formed by this layer should be carefully controlledto cause the molten ribbon material to adhere to a portion of it only(as shown in FIG. 4B). In applications using molten polysilicon, forexample, it is anticipated that contact angles with silicon of betweenabout 15 and 120° degrees should produce satisfactory results. Suchangles of greater than 25 degrees may produce even better results.

Among other ways, the nonwetting layer 32 may be formed by CVDprocesses, dip coating or other methods. For example, the base stringportion 26 may be CVD coated by applying electrical contacts in adeposition chamber while it is being fed through the chamber—thusheating the base string portion 26 itself. Alternatively, the basestring portion 26 may be heated by induction heating through thechamber.

Related techniques for implementing this step include:

-   -   a sol gel dip for silica or alumina oxide or silicon oxycarbide        either at the end of a CVD furnace or during rewind,    -   a CVD nonwetting coating deposited by heating quartz from the        outside and induction heating the base string portion 26,    -   spray-on deposition with a polymer binder that subsequently        would be burned off,    -   shaking particles onto a base string portion 26 or tow and then        baking the into the base string portion or tow, and    -   coating with base string portion 26 with refractory slurry        (e.g., silicon dioxide) or liquid and then burning off residual.

Prior to performing step 504, some embodiments form a handling layer 34that is separate from the produced nonwetting layer 32, as discussedabove. Accordingly, in such an embodiment, the nonwetting wetting layer32 substantially covers the handling layer 34. More specifically, thenonwetting layer 32 covers the outer, circumferential surface of thehandling layer 34.

It then is determined at step 506 if the coated string 12 has filamentsextending through the nonwetting layer 32 (such filaments are referredto herein as “whiskers”). This can occur, for example, when a tow offilaments forms the core 28. If the coated string 12 has whiskers, thenthe process shaves them off at step 508. The process then may loop backto step 504, which re-applies the nonwetting layer 32.

Alternatively, if the string 12 has no whiskers, the process continuesto step 510, which provides the string 12 to the furnace 14 as shown inFIG. 2. At this point, for each ribbon crystal being formed, the processpasses two strings 12 through the furnace 14 and crucible 18, thusforming the string ribbon crystal 10 (step 512).

Accordingly, illustrative embodiments increase the grain sizes near thestring 12, thus improving electrical efficiency of the ribbon crystals.By using a technique for matching the coefficient of thermal expansionwith the ribbon material, the inventors were able to achieve this goalwithout increasing yield loss.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A ribbon crystal comprising: a string having a string coefficient ofthermal expansion and an outer surface; and a body comprising a bodymaterial having a body coefficient of thermal expansion that isgenerally matched to the string coefficient of thermal expansion, thestring outer surface being partially exposed.
 2. The ribbon crystal asdefined by claim 1 wherein the string comprises a substrate supporting arefractory layer, the string further comprising a handling layerradially outward of the refractory layer, the handling layer applying agenerally radially inward force to the refractory layer.
 3. The ribboncrystal as defined by claim 2 wherein the handling layer includes thestring outer surface.
 4. The ribbon crystal as defined by claim 2wherein the string outer surface is radially outward of the handlinglayer.
 5. The string as defined by claim 2 wherein the handling layerhas a contact angle with the crystal material of between about 15 and120 degrees.
 6. The ribbon crystal as defined by claim 1 wherein theexposed portion of the string is free of body material.
 7. The ribboncrystal as defined by claim 1 wherein the string outer surface has acontact angle with the body material of between about 15 and 120degrees.
 8. The ribbon crystal as defined by claim 1 wherein the bodymaterial comprises polysilicon.
 9. The ribbon crystal as defined byclaim 1 wherein the string comprises a refractory material supported onand substantially entirely covering a substrate.
 10. The ribbon crystalas defined by claim 1 wherein the string outer surface comprises atleast one of pyrolytic carbon, oxide, and nitride.
 11. The ribboncrystal as defined by claim 1 wherein the string outer surface has acontact angle with the body material of greater than about 25 degrees.12. The ribbon crystal as defined by claim 1 wherein the body materialcomprises one of silicon, silicon-germanium, gallium arsenide and indiumphosphide.